Which one of the following elements does not form triiodide on reacting with iodine?

Options

A. B
B. Tl
C. Al
D. Ga

Answer

To determine which element does not form triiodide when reacting with iodine, we need to analyze the behavior of each element in the boron family when combined with iodine.

### Step-by-Step Solution:

1. **Identify the Elements**: The elements in the boron family include boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium (Tl). For this question, we will focus on boron, aluminum, gallium, and thallium.

2. **Understand the Formation of Triiodide**: Triiodide (I3-) is formed when an element can react with iodine (I2) to create a compound of the form EI3, where E is the element in question.

3. **Analyze Boron (B)**: When boron reacts with iodine, it forms boron triiodide (Bi3). Boron can achieve a +3 oxidation state, which is stable, allowing for the formation of B I3.

4. **Analyze Aluminum (Al)**: Aluminum also reacts with iodine to form aluminum triiodide (AlI3). Aluminum is stable in the +3 oxidation state, thus it can form the triiodide.

5. **Analyze Gallium (Ga)**: Gallium similarly reacts with iodine to form gallium triiodide (GaI3). Gallium is stable in the +3 oxidation state, allowing for the formation of GaI3.

6. **Analyze Thallium (Tl)**: Thallium can exist in +1 and +3 oxidation states. However, thallium triiodide (TlI3) is unstable. Thallium is more stable in the +1 oxidation state, leading to the formation of thallium iodide (TlI) instead of TlI3.

7. **Conclusion**: Among the elements analyzed, thallium does not form a stable triiodide when reacting with iodine. Therefore, the correct answer is thallium (Tl).

### Final Answer:
Thallium (Tl) does not form triiodide on reacting with iodine.

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